The present invention relates generally to measurement devices, and more particularly to noninvasive measurement of blood glucose by photoacoustics.
An estimated 16 million Americans (approximately 7% of the total population in the United States) have diabetes, a disease which can cause severe damage to the heart, kidneys, eyes and nerves. Diabetics need to monitor their blood glucose levels frequently, often as much as six times a day, to maintain a proper level of insulin in their blood. Intense testing and treatment of diabetes can reduce the complications, including blindness, kidney failure and heart attack, by as much as 70%.
A well-known invasive procedure for monitoring blood glucose levels involves pricking the finger of a patient to obtain a blood sample, and analyzing it for glucose content by the use of an enzyme-based method. This invasive method, which is painful and has risk of infection, often prevents the patient from performing the needed frequent testing and treatment. Additionally, because finger-stick monitoring is an enzyme-based technique, the cost for this technique is high.
Techniques which rely on non-invasive monitoring of glucose generally utilize infrared or near infrared technology to noninvasively obtain optical signatures indicating the level of glucose. Some of these infrared techniques rely on direct photoacoustic generation methods for noninvasive monitoring. In direct photoacoustic generation methods, the acoustic wave is produced in a sample where the excitation beam is absorbed. For example, U.S. Pat. No. 5,348,002 to Caro discloses a device for measuring blood glucose which includes a light source for applying electromagnetic radiation to tissue under analysis and a transducer for detecting acoustic energy. The transducer is positioned on one side of the finger and the incoming electromagnetic wave impinges on the other side of the finger, opposite the transducer. This technique is generally unreliable because a tissue, such as a body part, is optically thick. The impinging electromagnetic energy is almost totally absorbed by the tissue. Consequently, the measured acoustic wave will respond to the total incident electromagnetic energy--not just the fraction absorbed by glucose.
The technique disclosed in Caro also fails to compensate for the adverse effects caused by the absorption of radiation by water, rather than the medium to be measured, such as glucose. The effect of strong water absorption is twofold. First, because tissue has a high percentage of water, water absorption can prevent a light beam from penetrating a sufficient depth through tissue. Second, water absorption can yield an acoustic signal which is overwhelming compared to that from glucose. In particular, when electromagnetic energy impinges on water at certain wavelengths, the water optically absorbs the electromagnetic energy, inducing a temperature rise and related pressure variations in the tissue. The pressure changes caused by water are transmitted to the transducer as a series of pulses or waves, thus overwhelmingly interfering with the measurement of glucose.
Another photoacoustic method for direct, non-invasive monitoring of glucose which also fails to address the adverse effects caused by the absorption of radiation by water is described in EP 0 282 234, which discloses a technique for measuring blood glucose utilizing a transducer for monitoring acoustic energy. In EP 0 282 234, a semiconductor laser operating in the wavelength range of about 1300 to 1580 nm is utilized to excite glucose in a blood stream to generate acoustic energy. At this wavelength range, water absorption can adversely affect glucose measurements. As with other direct photoacoustic techniques, for accurate measurements, the medium must be optically thin. Unfortunately, most tissue in any body part is optically thick.
Other recent noninvasive devices for monitoring blood glucose suffer from shortcomings as well. One such device, referred to as the "Dream Beam", developed by Futrex Medical Instrumentation, Inc. of Gaithersburg, Maryland and disclosed in U.S. Pat. Nos. 5,028,787, 5,077,376 and 5,576,544, includes a battery-operated box about the size of a television remote control designed to provide noninvasive glucose measurements with the use of infrared radiation. Infrared light, having a wavelength between about 600 and 1000 nm, is passed through a finger. This approach has failed to produce accurate measurements as well.
Another noninvasive device, referred to as the "Diasensor 1000", developed by Biocontrol Technology, Inc. of Pittsburgh, Pa. and disclosed in U.S. Pat. No. 5,070,874, has failed to produce accurate results as well. In this device, a tabletop spectrophometer is designed to recognize a person's glucose patterns through the use of a light beam that passes through the skin of the forearm into the blood and is then reflected back to a sensor. A microprocessor is intended to interpret the data and calculate the blood glucose level. This reflection technique suffers from numerous shortcomings, including a small return signal, scattering from tissue (which reduces the signal and increases fluctuation) and interferences by strong background light.
As of date, no noninvasive glucose monitors, including the devices discussed above, have been approved by the Federal Drug Administration. This is mainly because current methods, which rely in large part on optical transmission or reflection, generally do not have sufficient sensitivity. In particular, absorption by glucose molecules is extremely weak compared to other blood constituents. Consequently, the return signal which is generated by other blood constituents is overwhelming compared to that by glucose, resulting in inaccurate measurements. Additionally, these optical techniques are severely limited by noise induced by light scattering through the tissue and cell walls. A sophisticated chemometric data process algorithm is often required to suppress such noise.
What is needed therefore is an apparatus and method for monitoring blood glucose which is painless, noninvasive, accurate and economical.